#include <iostream>
using namespace std;

int n,m,i,j,a,b,head,tail,ans;
int graph[100][100];
int degree[100];
int len[100];
int queue[100];
int main() {
    cin>>n>>m;
    for(i=0; i<n; i++)
        for(j=0; j<n; j++)
            graph[i][j]=0;
    for(i=0; i<n; i++)
        degree[i]=0;
    for(i=0; i<m; i++) {
        cin>>a>> b;
        graph[a][b]=1;
        (degree[b]++);
    }
    tail = 0;
    for (int i=0; i<n; i++)
        if(!degree[i]) {
            queue[tail]=i;
            tail++;
        }
    head= 0;
    while (tail<n) {
        for (i=0; i<n;  i++)
            if (graph[queue[head]][i]== 1) {
                (degree[i]--);
                if (degree[i]==0) {
                    queue[tail] = i;
                    tail++;
                }
            }
        (++head);
    }
    ans = 0;
    for (i=0; i<n; i++) {
        a =queue[i];
        
        len[a]=1;
        for (j=0; j<n; j++)
            if(graph[j][a]==1&& len[j]+1>len[a])
                len[a]=len[j]+1;
        if((len[a]> ans))
            ans = len[a];
    }
    cout<<ans<<endl;
    return 0;
}